Recently, development of an optical semiconductor device integrating a semiconductor laser and an optical modulator has been continued for application to high-capacity high-speed optical fiber communication. In this optical semiconductor device, a distributed-feedback semiconductor laser (hereinafter referred to as DFB laser) is operated by direct current, and the amount of the optical absorption a the laser light radiated from this laser is changed in an electric-field absorption type optical modulator by electric-field modulation, which modulator is disposed in the laser emission direction of the DFB laser, thereby performing as a high-speed intensity modulator. In comparison with a conventional direct modulation method which directly changes a driving current of a semiconductor laser, in this optical semiconductor device employing an electric-field absorption type modulator, wavelength chirping of the laser light is reduced, resulting in advantages in high-speed long-distance optical communication.
A description is given of a prior art optical semiconductor device integrating a DFB laser and a light absorption type modulator, which is illustrated in "InGaAs/InGaAsP MQW Electroabsorption Modulator Integrated with a DFB Laser Fabricated by Band-Gap Energy Control Selective Area MOCVD", IEEE J. Quantum Electron., Vol. 29, pp.2088-2096, 1993 by M. Aoki et al. FIG. 5 is a perspective view of the prior art optical semiconductor device, a portion of which is sectioned. In the figure, reference numeral 2 designates an n-type InP substrate, numeral 3 designates a bottom surface electrode, numeral 4 designates a light absorption layer of the optical modulator, numeral 7 designates a top surface electrode, numeral 8 designates a semi-insulating Fe-doped InP layer, numeral 9 designates an n-type InP hole blocking layer. Reference numeral 11 designates an active layer of the DFB laser, numeral 12 designates a diffraction grating, numeral 14 designates a mesa-shaped waveguide, numeral 35 designates a p-type InP upper cladding layer, numeral 101 designates the DFB laser, and numeral 102 designates the optical modulator.
FIGS. 6(a) and 6(b) are diagrams illustrating the prior art optical semiconductor device. FIG. 6(a) is a schematic view of a cross-section when the optical semiconductor device is sectioned through a broken line 6b--6b in the figure and along a plane parallel to the mesashaped waveguide. In the figures, the same reference numerals as in FIG. 5 designate the same or corresponding parts. Reference numeral 36 designates a region between the optical modulator 102 and the DFB laser 101.
A description is given of the structure of the prior art semiconductor device. The DFB laser 101 with the diffraction grating 12 under the active layer enables a stable laser oscillation with a single wavelength. The active layer 11 of the DFB laser 101 and the light absorption layer 4 of the optical modulator 102 comprise a continuous InGaAs/InGaAsP multi quantum well layer (hereinafter referred to as MQW layer). The MQW layer is thicker in the DFB laser 101 than in the optical modulator 102. The width of each quantum well included in this layer is larger in the DFB laser 101 than in the optical modulator 102. Consequently, the difference in energy between ground levels of the conduction band and the valence band in the DFB laser 101 is smaller than that in the optical modulator 102. Therefore, when no bias voltage is applied to the optical modulator 102, light from the DFB laser 101 is not absorbed in the light absorption layer 4 of the optical modulator 102. However, when a reverse bias voltage is applied to the optical modulator 102, the light is absorbed due to the quantum-confinement Stark effort (QCSE). Therefore, light emitted from the DC-operating DFB laser 101 can be modulated by varying a bias voltage applied to the optical modulator 102.
Moreover, to bury the mesa-shaped waveguide, a semi-insulating Fe-doped InP layer 8 and an n-type InP hole blocking layer 9 are disposed at both sides of the waveguide which comprises a continuous MQW structure consisting of the light absorption layer 4 and the active layer 11, an upper cladding layer 35 disposed above the MQW structure, and a lower InP cladding layer beneath the MQW structure (not shown). The InP layer 8 and the hole blocking layer 9 serve as a current blocking structure, reducing the threshold current and improving the efficiency of the DFB laser. Since in the InP Fe is in a deep acceptor level, the semi-insulating Fe-doped InP cladding layer 8 can prevent electrons from diffusing from the n-type InP substrate 2, and the n-type InP hole blocking layer 9 can prevent holes from diffusing from the upper p-type InP cladding layer 35.
As shown in FIGS. 6(a) and 6(b), an interface between the n-type InP hole blocking layer 9 and the upper p-type InP cladding layer 35 defines a pn junction, and the junction capacitance C.sub.1 is too large to be negligible for high speed operation of the optical modulator. The junction capacitance C.sub.3 in the DFB laser 101 is also as large as C.sub.1. On the other hand, the capacitances C.sub.2 and C.sub.4 between the n-type InP hole blocking layer 9 and the n-type InP substrate 2 are significantly smaller than C.sub.1 or C.sub.3 because, between these layers, there is interposed a thick semi-insulating Fe-doped InP layer 8. Since in the InP the mobility of electrons is considerably larger than that of holes, the electrical resistance of the n-type InP hole blocking layer 9 is low. Therefore, when the hole blocking layer is continuous through the optical modulator 102 and the DFB laser 101, there occurs a mutual interference between the modulator 101 and the DFB laser 102, the capacitance C.sub.3 becomes associated with the capacitance C.sub.1, and a parasitic capacitance of the optical modulator is increased, thereby impeding modulation at high frequencies. That is, the modulation bandwidth is narrow. In order to solve these problems, in the hole blocking layer 9 a portion 36 between the optical modulator 102 and the DFB laser 101 is removed by etching as shown in FIG. 6(b). In this etching, however, it is difficult to control etching depth, and the surface after the etching is rough.
FIGS. 7(a) and 7(b) illustrate another prior art optical semiconductor device in which the mutual interference is reduced between the modulator and the DFB laser without the etching-removal process. FIG. 7(a) is a cross-sectional view illustrating a whole structure of another prior art optical semiconductor device, and FIG. 7(b) is a partially cutaway view of the above device illustrating the inner structures of the modulator and the laser. This optical semiconductor device is disclosed in "Novel Current Blocking Structure for High Speed EA Modulator/DFB LD Integrated Light Source", I00C 95, Technical Digest, Vol.4, pp.60-61, 1995, by Y. Miyazaki et al.. The optical semiconductor device has a structure similar to that of the prior art device as described above, except that the n-type InP hole blocking layer 9 is interposed between two semi-insulating Fe-doped InP layers. In the figures, the same reference numerals as in FIG. 5 designate the same or corresponding parts, and reference numeral 1 designates an optical semiconductor device, numeral 5 designates a p-type InP second upper cladding layer, numeral 6 designates a p-type InGaAs contact layer, numeral 10 designates an upper semi-insulating Fe-doped InP layer, numeral 29 designates a protective film comprising an insulating film, and numeral 15 designates a mesa.
FIG. 8(a) is a cross-sectional view of the semiconductor device shown in FIGS. 7(a) and 7(b), and FIG. 8(b) is a schematic view of a cross section when the device is sectioned through a broken line 8b--8b and along a plane parallel to a mesa-shaped waveguide. In the figures, the same reference numerals as in FIGS. 7(a) and 7(b) designate the same or corresponding parts. Reference numeral 26 designates a p-type InP first upper cladding layer.
This semiconductor device has a structure similar to that of the conventional semiconductor device shown in FIG. 5, except that the upper semi-insulating Fe-doped InP layer 10 and the lower semi-insulating Fe-doped InP layer 8 are used in place of the conventional semi-insulating Fe-doped InP layer; the n-type InP hole blocking layer 9 is interposed between the layers 8 and 10, and a portion between the optical modulator 102 and the DFB laser 101 is not etched.
As shown in FIG. 8(b), the interface between the n-type InP hole blocking layer 9 and the upper semi-insulating Fe-doped InP layer 10 of the optical modulator 102 is not a pn junction. The junction capacitance C.sub.A between the p-type second cladding layer 5 and the hole blocking layer 9 is smaller than the capacitance C.sub.1 of the optical modulator 102 in the above prior art optical semiconductor device shown in FIG. 5(b).
Moreover, the junction capacitance C.sub.C of the DFB laser 101 is as small as C.sub.A. Thus, since the spacing of the pn junction, which is formed by the upper cladding layer 5 and the hole blocking layer 9, is wider than in the above prior art, the junction capacitance between these layers is reduced, so that the parasitic capacitances of the individual elements are reduced. This allows the device to operate at a frequency higher than in the first described prior art device. Even if the hole blocking layer 9 is continuous through the optical modulator 102 and the DFB laser 101 and the junction capacitance C.sub.A becomes associated with the capacitances C.sub.C and C.sub.D through the electrical resistance of the hole blocking layer 9, the capacitances C.sub.A and C.sub.C are considerably smaller than previously described, to reduce the mutual interference between the optical modulator 102 and the DFB laser 101. Therefore, no etching is required for a portion between the optical modulator 102 and the DFB laser 101.
Thus, in another prior art optical semiconductor device shown in FIGS. 7(a) and 7(b), it is possible to obtain superior element isolation characteristics without removing a portion between the laser 101 and the optical modulator 102 by etching, and to widen modulation bandwidth.
However, for an optical semiconductor device integrating two or more optical semiconductor elements that function differently, especially such as the prior art optical semiconductor device integrating an optical modulator and a DFB laser, it is necessary to design a structure that allows the elements of the device to operate with sufficient performances at the same time. In the case of the prior art device, it is required to have a structure in which parasitic capacitances between the optical modulator and the DFB laser is reduced, the mutual interference between the respective elements is reduced, and the basic light output-current characteristics (hereinafter referred to as P-I characteristics) is improved when the DFB laser oscillates. Especially, as shown in FIG. 7(a), in an optical semiconductor device integrating an optical modulator and a DFB laser which further includes, between two semi-insulting Fe-doped InP layers, a hole blocking layer that is continuous through the optical modulator and the DFB laser, the hole blocking layer and other layers of the optical modulator and the DFB laser are not separated by etching or the like and the hole blocking is conductive. Therefore, it is difficult to investigate exactly how the respective elements interfere each other, and to determine the design of the device by taking account of characteristics of the individual elements to realize the optimum optical semiconductor device. Thus, it is impossible to provide an optical semiconductor device in which its individual elements can operate with the utmost performances at the same time.